The use of electrochemical intercalation permits reversible energy storage through diffusive processes. Intercalation refers to the process wherein molecules, atoms, or ions are reversibly inserted in lattice vacancies, or between the layers of a lattice, in solid materials. The solid polymer electrolyte lithium-reversible battery is the product of research on solid ionic conductors and intercalation electrodes. It combines the use of thin-film lithium-ion-conducting polymer electrolytes, with lithium-ion-reversible electrodes. This all-solid rechargeable electrochemical cell is made of two lithium reversible electrodes, one acting as a source of lithium ions during discharge, the other as a lithium ion sink. The two electrodes are separated by a thin polymer electrolyte acting as a lithium ion carrier. The electrolyte normally contains an inorganic ionic salt and a solvent. The process is reversed during recharge. The lithium ion source can be a lithium metal foil (or lithium alloy); a low-potential lithium ion insertion material (e.g. WO.sub.2); or a lithium n-doped conjugated polymer. The lithium ion sink is usually a lithium ion insertion compound (TiS.sub.2, V.sub.6 O.sub.13, MoO.sub.2, etc.); another lithium reducible transition metal compound such as FeS.sub.2, NiS.sub.2 ; or a p-doped conjugated polymer. The lithium metal foil anode and the lithium ion intercalatable (insertion) cathode combination is usually seen as the best choice from the point of view of energy density and discharge/recharge cyclability capacity.
In a solid polymer electrolyte battery, the lithium ion carrier is generally obtained by dissolving a lithium salt (LiClO.sub.4, LiCF.sub.3 SO.sub.3) in a solvating aprotic polymer such as poly (ethylene) oxide with a solvent (propylene carbonate, ethylene carbonate, tetrahydrofurane, glymes, dioxolane, etc.).
Buckminsterfullerene, C.sub.60, was first synthesized in 1985. Five years later, a process for making it in large quantities was devised (Nature, 1990, 347, 354; Chem. Phys. Lett., 1990, 170, 167). The molecule has a structure like no other. It is a C.sub.60 molecule composed entirely of carbon. It is a hollow shell, very nearly spherical. It is only the third form of pure carbon known in nature. The other two being diamond and graphite. Geometrically, the "bucky ball" is a truncated regular icosahedron. The C.sub.60 member of the fullerene family is most prominent, but there are other members of the family. C.sub.70 fullerene has the oblong shape of a rugby football, and there are smaller fullerenes, such as C.sub.44. Partially hydrogenated fullerenes are also known, such as C.sub.60 H.sub.36, which maintain the geometric structure of C.sub.60. Haufler, J. Phys. Chem., 1990, 94, 8634, reports the ease of reduction of C.sub.60, and suggests it should be possible to electrochemically produce stable "buckide" salts. He predicts applications such as new materials, perhaps even extending to a new class of rechargeable batteries.
Chabre et al., J. Am. Chem. Soc., 1992, 114, 764-766, demonstrated the electrochemical intercalation of lithium into solid C.sub.60 using solid electrochemical cells and polymer electrolyte. Their cell was constructed with metallic lithium negative electrodes, a P(EO).sub.8 LiClO.sub.4 polymer electrolyte film, and a composite positive electrode containing 60% active material and 40% electrolyte by volume. The positive electrode was cast from acetonitrile suspension on a stainless steel current collector. The cells were operated at 80.degree. C. in order to maintain the electrolyte in the high ionic conducting amorphous phase.
The ionic conductivity of solid polymer, PEO, electrolyte in lithium/fullerene batteries is a major limitation to operation at ambient temperatures, especially for applications under low temperature conditions. Warming the batteries from ordinary temperatures to minimum operating temperatures of more than 60.degree. C. requires large and wasteful expenditures of energy.
It would be advantageous if a solid polymer electrolyte/fullerene battery could operate efficiently at lower temperatures.
It would also be advantageous if a solid polymer electrolyte fullerene battery operated in numerous discharge/recharge cycles with high cumulative capacity.